Trailers to be towed by tractors are well known. The requirements for towing such tractors may vary quite considerably in accordance with whether the trailer is to be pulled over a relatively smooth roadway, or is to travel on crude roads or off road terrain. Many such trailers must be towed on either smooth roads or on rough roads or rough road terrain. For example, military supplies including tanks are likely to move in part over smooth roads, and in part over rough roads or off roads completely. The same is true of trailers to be towed in the area of oil wells and the like. The requirements for a trailer to be towed on a smooth road or to be towed on a rough road or off the road are frequently conflicting. It is desired to have the bed as low as possible for simplified loading, and for carrying of cargo under highway bridges. However, such a trailer may also be towed on rough roads or on off road terrain, and this requires a substantial spacing between the tops of the tires of such a trailer and the bottom of the trailer bed.
The requirements for axial spacing of sets of wheels also are antithetical. For towing on relatively smooth, level roads it is desired that the axial spacing and overall axial distance covered by wheels should be relatively small in order to stay on the road without protruding over the center line. On the other hand, when the same trailer is towed on rough roads or on off road terrain there may be considerable side-to-side tipping forces, and it therefore is desirable to have as great a spacing as possible in an axial direction between corresponding wheels or sets of wheels.